Interspecific variation in predation patterns of stoats and weasels in an alpine conservation programme

Conservation programmes in New Zealand often suppress populations of a single invasive predator for the benefit of threatened avifauna. However, the establishment of whole guilds of invasive species has created complex competitor and predator-prey relationships, including some well-described trophic cascades. Trap networks designed to target stoats (Mustela erminea) are poorly optimised to supress a population of weasels (M. nivalis), and may contribute to periodic spikes in weasel numbers due to decreased interspecific competition and aggression. The consequences of stoat removal and possible weasel release have received little attention. In this small-scale pilot study, we used C13 and N15 stable isotopes to examine diet and relative trophic position of eight weasels and 20 stoats caught in an alpine ecosystem. We explored three dietary models as a framework with which to examine trophic patterns in feeding behaviour of the two mustelid species and potential impacts on native species. The models suggest that, in this specific scenario, weasels preyed at a higher trophic position than stoats and consumed a greater proportion (per capita) of vulnerable taxa like lizards and passerines. We cautiously highlight the potential for negative outcomes for some native taxa in scenarios in which weasels are released from competition with stoats.